Typically, a process unit having a photosensitive drum where a toner image is borne by development of electrostatic latent image is removably mounted in an image forming apparatus such as a laser printer. The photosensitive drum is disposed to be in contact with and to oppose to a transfer roller, and transfers a toner image onto a sheet while the sheet passes between the photosensitive drum and the transfer roller, so that the image is formed on the sheet.
However, when a gap occurs between the sheet and the photosensitive drum in an upstream side of a sheet conveying direction with respect to a transfer position where the photosensitive drum and the transfer roller are in contact with each other, discharge may occur in the gap. When such discharge occurs, particularly in a case of thin sheet, a discharge shape like a spot is shown, which is called a penetration which penetrates the sheet.
Accordingly, it has been proposed that a guide member for guiding a front end of the sheet to be closer to a surface of the photosensitive drum be mounted in the upstream side of the sheet conveying direction with respect to the transfer position.
FIGS. 31A-32C show the sheet transport states in the conventional process cartridge 90 in a stepwise manner. In this process cartridge 90, a photosensitive drum 91, and a transfer roller 92 in contact with and opposed to the photosensitive drum 91 are held by a case 93. A guide member 95 is installed in the case 93 for guiding the sheet 94 to the transfer position where the photosensitive drum 91 and the transfer roller 92 are in contact with each other.
A base end 96 (i.e., an end of the upstream side of the conveying direction of the sheet 94, which will be used with the same meaning in the following description) of the guide member 95 is fixed to the case 93 such that the guide member 95 is inclined upward toward the downstream from the upstream side of the conveying direction of the sheet 94 (Hereinafter, the downstream or upstream side of the conveying direction of the sheet 94 will be referred to as a simply downstream or upstream in some cases). A leading end 97 (i.e., an end of the downstream side of the conveying direction of the sheet 94, which will be used with the same meaning in the following description) of the guide member 95 is closely disposed to the photosensitive drum 91 in the upstream with respect to the transfer position.
As shown in FIGS. 31A and 31B, when the sheet 94 is transported to the process cartridge 90, the front end of the sheet 94 is guided toward the downstream along a top surface of the guide member 95 to reach the leading end 97 from the base end 96 of the guide member 95, and then guided to be in contact with the upstream side of the transfer position in the photosensitive drum 91.
The sheet 94 is then closely adhered to the photosensitive drum 91 in the upstream side of the transfer position from its front end and is guided to a location near its rear end while being in closely adhered to the photosensitive drum 91 as shown in FIG. 32A. Accordingly, the sheet 94 does not allow the gap to occur between the sheet and the photosensitive drum 91 in the upstream side of the transfer position, which thus suppresses the discharge from occurring therebetween. As a result, the discharge having a spot shape may be prevented from occurring in the sheet 94.
However, in accordance with this guide member 95, when the rear end of the sheet 94 passes the leading end 97 of the guide member 95, the rear end of the sheet 94 falls downward from the leading end 97 of the guide member 95 to be guided to the transfer position as shown in FIGS. 32B and 32C. Accordingly, the rear end of the sheet 94 is fluctuated to cause a transfer defect in the rear end.
Accordingly, it has been proposed that first and second guide members formed of polyester film having the same thickness be disposed in parallel with each other in the upstream side of the conveying direction of a transfer sheet with respect to an image carrier (e.g., see JP-A-8-036313). According to this proposal, a rear end of the transfer sheet which has passed the first guide member is guided to the image carrier by the second guide member disposed near the transfer position. As a result, the transfer defects in the rear end of the transfer sheet are reduced.
However, a thin sheet such as a weak plain sheet is apt to be discharged, so that it needs to be closely adhered to the photosensitive drum in the upstream side of the transfer position to prevent such discharge as described above. In the meantime, a thick sheet such as a strong postcard is not apt to be discharged as compared to the thin sheet, and needs to be correctly transported as much as possible because it causes bending when the thick sheet is closely adhered to the photosensitive drum as described above.
However, according to the configuration disclosed in JP-A-8-036313, the second guide member is disposed in parallel with the first guide member, so that the transfer sheet which has passed the first guide member is in contact with the second guide member disposed at the same inclination angle as the first guide member at a big angle. In this case, a resistant force of the second guide member against the transfer sheet increases so that the transfer sheet is strongly pressed toward the image carrier by the second guide member. Accordingly, in a case of such thick sheet, the transfer sheet is bent by the pressing to cause the transfer defect.
In addition, according to the configuration disclosed in JP-A-8-036313, since a second guide member is provided parallel to a first guide member, a transfer sheet passing through the first guide member contacts the second guide member, which is provided separately from the first guide member and is inclined in the same direction as that of the first guide member, from the same direction as the inclined direction.
In this case, the transfer sheet is pressurized by the second guide member from a direction approaching an image carrier toward an upstream side of a conveying direction of a transfer medium. Thus, a thick transfer sheet is bent due to the pressurization, leading to a transfer failure.